The present invention relates to a method of joining different metal materials by friction welding, and especially to a method of joining a rod material made of Ti alloy with a material made of Tixe2x80x94Al intermetallic compound by friction welding to make a poppet valve for an internal combustion engine.
A Ti alloy poppet valve has lower specific gravity and smaller inertial mass, and provides advantages that inputs, mileage and engine performance are increased.
Heat resistant temperature of Ti alloy is about 500xc2x0 C., and it is difficult to make an exhaust valve for high-combustion temperature engines such as internal-tube direct-jetting or lean-burn engines which require high temperature strength.
To overcome the disadvantage, a Ti alloy valve is disclosed in Japanese Patent Pub. No.62-197610, in which a valve head which requires high heat resistance is made of Ti-6Al-2Sn-4Zr-2Mo and a valve stem to which lower thermal load is applied is made of Ti-6Al-4V. They are joined by welding. In another method, a poppet valve is made of Tixe2x80x94Al intermetallic compound.
In the former, the poppet valve which has a valve head made of high heat resistant Ti alloy provides low durability and reliability if it is used as a high speed and high load exhaust valve in which temperature of a valve head reaches to 800xc2x0 C.
The poppet valve made of Tixe2x80x94Al intermetallic compound has lower specific gravity than that of Ti alloy poppet valve and provides advantage for lightening the valve itself. It has high temperature strength similar to those of heat resistant steel and Ni heat resistant alloy such as Inconel, and can be used as an exhaust valve.
However, it is difficult to form it by ordinary forging, and it is required to form it by casting. Thus, the number of valve manufactured per one casting step is very small to decrease productivity and increase cost.
Furthermore, a poppet valve has a valve head at one end of a longer valve stem. In the above casting process, casting defects such as cavities are liable to occur in a smaller poppet valve. Thus, it is necessary to examine all the products by non destructive inspection using X-ray, which results in increase in cost.
The inventors made a valve head which requires high heat resistance, from Tixe2x80x94Al intermetallic compound by casting to overcome casting defects, and a valve stem from Ti alloy such as Ti-6Al-4V, and joined the valve head with the valve stem.
However, between Tixe2x80x94Al intermetallic compound of the valve head and Ti alloy of the valve stem, there are differences in structure and melting point. Thus, sufficient joining strength cannot be achieved by welding such as brazing. Further, Ti is very active and liable to be oxidized at high temperature to form oxidization film at welded boundaries to cause rupture.
To solve the disadvantages, the valve head is joined to the valve stem by friction welding. In ordinary friction welding, Tixe2x80x94Al intermetallic compound is higher in high temperature strength than Ti alloy. Therefore, Ti alloy is plastically deformed prior to deformation of intermetallic compound and joining cannot be suitably attained.
In view of the disadvantages, it is an object to provide a method of joining different metal materials made of Ti alloy and Tixe2x80x94Al intermetallic compound by friction welding to increase tensile strength.
According to one aspect of the present invention, there is provided a method of joining different metal materials by friction welding, comprising the steps of: holding a first material made of Ti alloy by a rotating chuck of a friction welding machine, said first material having a smaller diameter portion and a larger diameter portion at an end; holding a second material made of Tixe2x80x94Al intermetallic compound by a stationary chuck of the friction welding machine, the smaller diameter portion of the first material being substantially equal in diameter to an end of the second material; pressing the end of the second material into the larger diameter portion at the end of the first material; rotating the first material in the rotating chuck at high speed; and stopping rotation of the first material to join the end of the second material the end of in the first material firmly.
According to another aspect of the present invention, there is provided a method of joining different metal materials by friction welding, comprising the steps of: providing a first material made of Ti alloy and a second material made of Tixe2x80x94Al intermetallic compound; heating an end of the second material; and joining an end of the first material with the end of the second material by friction welding.